1. Field of the Invention
This invention relates to a method for recording and reproducing information using optical memory devices incorporating photochromic materials, and particularly to a method for recording and reproducing information by which information stored in the optical devices can be reproduced using commercially available CD (Compact Disk) player systems.
2. Description of the Related Art
There have been developed a magnetooptical disk, a phase-change type disk and the like as optical memory devices for recording, erasing and reproducing information by utilizing light. In the magnetooptical disk, information is recorded in a manner such that a laser beam is applied onto a magnetic recording film to raise the temperature thereof up to about the Curie point for reducing antimagnetic properties thereof, and then an external magnetic field, which can be inverted in accordance with ON-OFF signals of information, is applied thereonto so that the information is recorded. On the other side, in the phase-change type disk, information is recorded in a manner such that a laser beam, which is ON-OFF controlled in accordance with ON-OFF signals of information, is applied onto a phase-change type recording film to fuse crystals of the laser-applied portion thereof or to rapidly cool the fused crystals for changing the phase thereof from crystalline state to amorphous state or vice versa, thereby recording the information.
By the way, CD-player systems for home use are now prevailing in developed countries, and accordingly it would be convenient if the information recorded in the aforesaid rewritable type optical memory devices can be reproduced with such CD-player systems.
With the aforesaid magnetooptical disk and phase-change type disk, however, information is recorded by transforming laser light into thermal energy, and therefore the laser light is substantially absorbed by the recording film, resulting in theoretically weakened reflected light. For this reason the CD-player systems of which standard requires optical memory devices to have the reflectance of 70% or more with respect to the light of 780 nm wavelength have a difficulty in reproducing the information recorded in the above optical disks.
For increasing the reflectance, Japanese Unexamined Patent Publication No. 12428/1982 have disclosed a magnetooptical disk which is so arranged that a thin magnetooptical film is formed over a light-reflecting member such as a Au film having a sufficient reflectance with respect to the light for reproducing information in order to enlarge the Kerr rotation angle. Despite such an arrangement, the reflectance remains as low as about 30%, and thus the S/N ratio is insufficient for reproducing information.
On the other hand, heretofore known is a method for recording and reproducing information in which a recording film is made of a binder resin containing a photochromic material dispersed therein, and light is applied onto the recording film to turn it into a chromatic state thereby recording and reproducing information. Also, various photochromic materials to be used in such a method have been proposed (Japanese Unexamined Patent Publication No. 122577/1984; "Photomemory Materials: Present State and Future Prospect in Japan", Proc. Int. Symp. on Optical Memory, 1987, Japanese Journal of Applied Physics, Vol. 26(1987) Supplement 26-4).
However, no one knows that the above method has realized an access to commercially available CD-player systems.